In recent years, a semiconductor integrated circuit, particularly a microcontroller for control use, is required to have high-speed processing capability, and at the same time, with low power consumption. In order to meet a requirement for processing at higher speed, such an integrated circuit becomes more minutely fabricated. As each transistor size becomes smaller, an operation voltage tends to become lower. This requires a semiconductor integrated circuit to be provided with an internal power supply which supplies a lower voltage in comparison with a power supply voltage prepared by an external circuit. For this purpose, in such a semiconductor integrated circuit, there is embedded a direct current (DC) voltage regulator which generates an internal supply voltage stepped down from an external supply voltage. Controlling the semiconductor integrated circuit to operate with a decreased internal supply voltage enables to reduce power consumption.
Further, in order to obtain lower power consumption, a variety of power saving modes may often be provided in a semiconductor integrated circuit such as a microcontroller. As one example, a semiconductor integrated circuit is so configured as to shift to a standby mode when a state of no operational instruction continues for a predetermined period. Clock signal supply is suspended during such a standby mode, which generally suspends a part of operation in an internal circuit of the integrated circuit.
In the conventional semiconductor integrated circuit, internal supply voltage is so controlled as to keep constant. For example, the internal supply voltage is controlled to maintain in a constant voltage irrespective of a high-speed operation mode or a low-speed operation mode. In the high-speed operation mode, an operation clock signal is set to have a higher frequency, which produces larger power consumption in the internal circuit, while in the low-speed operation mode, the operation clock signal frequency is set lower, which produces smaller power consumption in the internal circuit. Thus, when controlling the internal circuit to operate at lower speed by decreasing the supplied clock signal frequency, a substantial power saving mode can be attained. In such a manner, actual power saving is achieved as a whole.
However, there has been arising requirement for further power saving in recent years. In particular, for example, in case of a portable device the control unit of which is driven by batteries, further reduction of power consumption is strongly desired.